The present invention relates generally to a telemetry system for transmitting data from a downhole drilling assembly to the surface of a well during drilling operations. More particularly, the present invention relates to a pilot operated mud pulsing valve for use in a measurement while drilling ("MWD") system or a logging while drilling ("LWD") system to transmit downhole measurements to the surface of the well during drilling operations through the medium of the drilling fluid. Still more particularly, the present invention relates to a mud pulsing valve capable of generating multiple bits of data in a single pulse to increase the bandwidth of the mud pulse telemetry system.
Modern petroleum drilling and production operations demand a great quantity of information relating to parameters and conditions downhole. Such information typically includes characteristics of the earth formations traversed by the wellbore, in addition to data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as "logging," can be performed by several methods. Oil well logging has been known in the industry for many years as a technique for providing information to a driller regarding the particular earth formation being drilled. In conventional oil well wireline logging, a probe or "sonde" housing formation sensors is lowered into the borehole after some or all of the well has been drilled, and is used to determine certain characteristics of the formations traversed by the borehole. The sonde is supported by a conductive wireline, which attaches to the sonde at the upper end. Power is transmitted to the sensors and instrumentation in the sonde through the conductive wireline. Similarly, the instrumentation in the sonde communicates information to the surface by electrical signals transmitted through the wireline.
The problem with obtaining downhole measurements via wireline is that the drilling assembly must be removed or "tripped" from the drilled borehole before the desired borehole information can be obtained. This can be both time-consuming and extremely costly, especially in situations where a substantial portion of the well has been drilled. In this situation, thousands of feet of tubing may need to be removed and stacked on the platform (if offshore). Typically, drilling rigs are rented by the day at a substantial cost. Consequently, the cost of drilling a well is directly proportional to the time required to complete the drilling process. Removing thousands of feet of tubing to insert a wireline logging tool can be an expensive proposition.
As a result, there has been an increased emphasis on the collection of data during the drilling process. By collecting and processing data during the drilling process, without the necessity of removing or tripping the drilling assembly to insert a wireline logging tool, the driller can make accurate modifications or corrections, as necessary, to optimize performance while minimizing down time. Designs for measuring conditions downhole and the movement and location of the drilling assembly, contemporaneously with the drilling of the well, have come to be known as "measurement-while-drilling" techniques, or "MWD." Similar techniques, concentrating more on the measurement of formation parameters, commonly have been referred to as "logging while drilling" techniques, or "LWD." While distinctions between MWD and LWD may exist, the terms MWD and LWD often are used interchangeably. For the purposes of this disclosure, the term MWD will be used with the understanding that this term encompasses both the collection of formation parameters and the collection of information relating to the movement and position of the drilling assembly.
Drilling oil and gas wells is carried out by means of a string of drill pipes connected together so as to form a drill string. Connected to the lower end of the drill string is a drill bit. The bit is rotated and drilling accomplished by either rotating the drill string, or by use of a downhole motor near the drill bit, or by both methods. Drilling fluid, termed mud, is pumped down through the drill string at high pressures and volumes (such as 3000 p.s.i. at flow rates of up to 1400 gallons per minute) to emerge through nozzles or jets in the drill bit. The mud then travels back up the hole via the annulus formed between the exterior of the drill string and the wall of the borehole. On the surface, the drilling mud is cleaned and then recirculated. The drilling mud is used to cool the drill bit, to carry cuttings from the base of the bore to the surface, and to balance the hydrostatic pressure in the rock formations.
When oil wells or other boreholes are being drilled, it is frequently necessary or desirable to determine the direction and inclination of the drill bit and downhole motor so that the assembly can be steered in the correct direction. Additionally, information may be required concerning the nature of the strata being drilled, such as the formation's resistivity, porosity, density and its measure of gamma radiation. It is also frequently desirable to know other down hole parameters, such as the temperature and the pressure at the base of the borehole, as examples. Once these data are gathered at the bottom of the bore hole, it is typically transmitted to the surface for use and analysis by the driller.
In LWD systems sensors or transducers typically are located at the lower end of the drill string which, while drilling is in progress, continuously or intermittently monitor predetermined drilling parameters and formation data and transmit the information to a surface detector by some form of telemetry. Typically, the down hole sensors employed in MWD applications are positioned in a cylindrical drill collar that is positioned close to the drill bit. The LWD system then employs a system of telemetry in which the data acquired by the sensors is transmitted to a receiver located on the surface. There are a number of telemetry systems in the prior art which seek to transmit information regarding downhole parameters up to the surface without requiring the use of a wireline tool. Of these, the mud pulse system is one of the most widely used telemetry systems for MWD applications.
The mud pulse system of telemetry creates acoustic signals in the drilling fluid that is circulated under pressure through the drill string during drilling operations. The information that is acquired by the downhole sensors is transmitted by suitably timing the formation of pressure pulses in the mud stream. The information is received and decoded by a pressure transducer and computer at the surface.
In a mud pressure pulse system, the drilling mud pressure in the drill string is modulated by means of a valve and control mechanism, generally termed a pulser or mud pulser. The pulser is usually mounted in a specially adapted drill collar positioned above the drill bit. The generated pressure pulse travels up the mud column inside the drill string at the velocity of sound in the mud. Depending on the type of drilling fluid used, the velocity may vary between approximately 3000 and 5000 feet per second. The rate of transmission of data, however, is relatively slow due to pulse spreading, distortion, attenuation, modulation rate limitations, and other disruptive forces, such as the ambient noise in the drill string. A typical pulse rate is on the order of a pulse per second (1 Hz). As shown for example in FIG. 1, the mud pulse signal is comprised of a pressure pulse at one of two amplitudes, indicating whether the mud pulser or "poppet" is open or closed. If the poppet is closed, a high pressure pulse is generated, to indicate, for example, a digital "1." If the poppet is opened, a digital "0" is indicated. The primary method of increasing the data rate of the transmitted signal is to increase the frequency f of the pulses. As the frequency f of the pulses increases, however, it becomes more and more difficult to distinguish between adjacent pulses because the resolution period is too short. The problem is that the period T for each individual pulse has decreased proportionately (T=1/f). The resolution therefore decreases, causing problems with detection of the adjacent pulses at the surface. Representative examples of mud pulse telemetry systems may be found in U.S. Pat. Nos. 3,949,354, 3,958,217, 4,216,536, 4,401,134, and 4,515,225.
Mud pressure pulses can be generated by opening and closing a valve near the bottom of the drill string so as to momentarily restrict the mud flow. In a number of known MWD tools, a "negative" pressure pulse is created in the fluid by temporarily opening a valve in the drill collar so that some of the drilling fluid will bypass the bit, the open valve allowing direct communication between the high pressure fluid inside the drill string and the fluid at lower pressure returning to the surface via the exterior of the string.
Alternatively, a "positive" pressure pulse can be created by temporarily restricting the downwardly flow of drilling fluid by partially blocking the fluid path in the drillstring. Whatever type of pulse system is employed, detection of the pulses at the surface is sometimes difficult due to attenuation and distortion of the signal and the presence of noise generated by the mud pumps, the downhole mud motor and elsewhere in the drilling system. Typically, a pressure transducer is mounted directly on the line or pipe that is used to supply the drilling fluid to the drill string. An access port or tapping is formed in the pipe, and the transducer is threaded into the port. With some types of transducers, a portion of the device extends into the stream of flowing mud where it is subject to wear and damage as a result of the abrasive nature and high velocity of the drilling fluid. In any case, the transducer detects variations in the drilling mud pressure at the surface and generates electrical signals responsive to these pressure variations.
Both the positive and negative mud pulse systems typically generate base band signals. In an attempt to increase the data rate and reliability of the mud pulse signal, other techniques also have been developed as an alternative to the positive or negative pressure pulses generated. One early system is that disclosed in U.S. Pat. No. 3,309,656, which used a downhole pressure pulse generator or modulator to transmit modulated signals, carrying encoded data, at acoustic frequencies to the surface through the drilling fluid or drilling mud in the drill string. In this and similar types of systems, the downhole electrical components are powered by a downhole turbine generator unit, usually located downstream of the modulator unit, that is driven by the flow of drilling fluid. These type of devices typically are referred to as mud sirens. Other examples of such devices may be found in U.S. Pat. Nos. 3,792,429, 4,785,300 and Re. No. 29,734. One problem with these mud siren type of modulators is the difficulty in retrieving and distinguishing the frequency modulated signals at the surface.
None of the prior art devices to date have been capable of providing an increased bandwidth signal in a form that provides easy delineation at the surface of the well.